PPVPN Working Group Michael Behringer Internet Draft Jim Guichard Expiration Date: July 2003 Cisco Systems Inc January 7th 2002 MPLS VPN Import/Export Verification draft-behringer-mpls-vpn-auth-01.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract Configuration errors on Provider Edge (PE) routers in MPLS VPN [RFC2547] networks can lead to security breaches of the connected VPNs. For example the PE could be mistakenly configured such that a connected Customer Edge (CE) router belongs to an incorrect VPN. Here we propose a scheme that verifies local and remote routing information received by the PE router before it installs new VPN routes into the VRF. The proposed changes affect only the PE routers. Table of Contents Internet Draft MPLS VPN Authentication January 2003 1. Conventions used in this document 2 2. Overview 2 3. Problem Statement 3 4. CE-CE Authentication 3 5. Supported PE-CE Routing Protocols 5 6. The UPDATE-authenticator attribute 6 7. IANA Considerations 6 8. Security Considerations 6 9. Acknowledgements 7 10. References 7 11. Author's Addresses 7 12. Full Copyright Statement 7 1. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC-2119]. 2. Overview The current MPLS standards do not provide any mechanism to determine whether an imported route on a PE router originated from the correct VPN. This opens a potential security hole where the MPLS Service Provider could mistakenly assign on a PE router the incorrect "route- target" values, thus inadvertently bringing a connected customer site into a wrong VPN. Misconfigurations of this type by the Service Provider are hard to detect, and a real issue in [RFC2547] networks. One possible solution to this problem is to mount IPsec on all CE routers, but this is often perceived as too "heavy-weight". This proposal aims to solve the problem of accidental misconfiguration of MPLS parameters on PE routers. The approach is to re-use the routing MD5 key, which is used for PE-CE authentication [RFC2082, 2154, 2385], also for the PE-PE routing authentication. Since a PE router can hold several virtual routing and forwarding instances (VRFs), the authentication between PEs will use the different MD5 keys, based on which VRF's routes need to be authenticated. BGP UPDATE messages between PE routers will include a new BGP attribute, hereby referred to as the "UPDATE-authenticator". This attribute contains a keyed HMAC MD5 signature of a locally generated draft-behringer-mpls-vpn-auth-01.txt page 2 Internet Draft MPLS VPN Authentication January 2003 per-VRF random number, using the MD5 key that is also used on this PE router for the PE-CE routing authentication of that VPN. The receiving PE router generates a keyed HMAC MD5 signature for the incoming UPDATE message using information from the "UPDATE- authenticator" attribute, and the routing key of the CE router that is to receive these routing updates. If the result is different from the signature value transmitted in the UPDATE-authenticator attribute, the routes within the UPDATE are not imported and a warning is logged. The proposal imposes some operational constraints to be workable: Routing with MD5 authentication [RFC2082, 2154, 2385] must be configured for all PE-CE links of a particular VPN, and all CEs of the same VPN must use the same MD5 key. If the Service Provider manages the CE routers on behalf of the customer, then downstream C routers must also use the same MD5 key. MD5 keys must be chosen to be unique to a VPN. 3. Problem Statement The current MPLS architecture as defined in [RFC2547] allows a three- step configuration process for route authentication: From the CE to the PE, from that PE to other PEs in the same MPLS network, and from the other PEs to the corresponding CEs. The link between these three steps depends on correct configuration of the MPLS core network, namely the correct configuration of "route-targets" on the PE routers. If an MPLS Service Provider inadvertently configures the wrong "route- targets", they may introduce a random CE router with the network behind it to another VPN. This error is hard to detect from within the customer's network. A mechanism is required which prevents routes from being passed into a PEÆs VRF, unless they are verified to belong to the VPN. Also in the case of misconfigurations, the Service Provider must be alerted so that the mistake can be rectified. 4. CE-CE Authentication This document proposes to re-use the MD5 key that is being used for PE- CE routing authentication. This has the advantage that no changes or software upgrades are necessary at the CE routers or within the VPN site. For this proposal to work each VPN MUST use the same MD5 key on all PE-CE links for routing authentication. The MD5 key MUST be unique to each VPN. The VPN customer configures thus all their CE routers with draft-behringer-mpls-vpn-auth-01.txt page 3 Internet Draft MPLS VPN Authentication January 2003 this MD5 key. The MPLS Service Provider configures the PEs also with this key on all links to the customers CEs. This proposal does not affect the CE-PE routing authentication, but the authentication MUST be used for this scheme to work. In normal [RFC2547] networks, the PE router that receives routes from a CE advertises these routes via MP-BGP to the other PEs, using a PE specific MD5 key. This is a problem as there is no linkage between the MD5 key used for PE-CE authentication and the one used for PE-PE authentication. This essentially introduces a 3-stage authentication process, with each stage disjointed from the other two stages. Using the mechanisms described within this document, the BGP UPDATE message, as defined in [RFC1771], is sent between PE routers (or BGP route reflectors), and carries a new UPDATE-authenticator attribute, which is handled as follows: 1. The PE will only export routes from a local VRF that is configured for MD5 authentication if it can confirm that at least 1 local route has been received and authenticated from each CE router that is attached to the VRF. This prevents any connected interface or static routes from leaking into other VPNs without first confirming the local configuration of the PE router. The exception to this rule is the 'Trusted PE' model (see section 5). 2. The PE will calculate a random number, referred to as the 'generator', for each VRF that is configured for authentication. Alternatively a combination of the local route-target values may be used to generate this number. This is implementation specific. 3. The PE sending an MP-BGP UPDATE calculates a keyed HMAC-MD5 signature, as defined in [RFC2104], over the 'generator', using the key of one of the CEs that is connected to the corresponding VRF. The result of this calculation is carried, along with the 'generator', within the UPDATE-authenticator attribute. 4. The UPDATE message is sent to any MP-BGP peers (other PE routers or BGP route reflectors). 5. The route targets in the MP-BGP update determine which VRF/s the UPDATE refers to, and these are used as normal to determine which PE routers will import which routes. 6. The receiving PE MUST not import a received prefix if it contains the UPDATE-authenticator attribute, and the local VRF is NOT configured for authentication. Likewise, the receiving PE MUST not import a draft-behringer-mpls-vpn-auth-01.txt page 4 Internet Draft MPLS VPN Authentication January 2003 received prefix if it does not contain the UPDATE-authenticator attribute, and the local VRF is configured for authentication. 7. The receiving PE uses the local MD5 key for the importing VRF to calculate a HMAC-MD5 signature, and compares the result to the value in the received "UPDATE authenticator" path attribute. This comparison is ONLY performed if the PE has at least 1 locally authenticated route from the CE router. 8. If the values are identical, the UPDATE is processed as normal and the routes are installed into the VRF. If not, the PE router MUST check if any further keys are available within the VRF. If they are then it should cycle through step 7 until either a) there are no more keys available, or b) a match is found. 9. If no match is found then the BGP table is updated but the routes are marked as 'not authenticated', and a warning is logged to indicate a misconfiguration to the Service Provider. A router MAY verify whether all MD5 keys for a given VRF are the same. If it does a warning message MUST be logged if it detects differences. In the case where the Service Provider manages the CE routers, the Service Provider must also configure the key at the CE routers and this should match with any downstream C routers within the customer site. If the C routers have a different key than the CE router then the CE will not authenticate any routes from within the site, and will therefore not advertise any routing information to the PE router. The PE router is thus able to use the previously described mechanisms and will not import/export any routes from/to the customers VRF. 5. Supported PE-CE Routing Protocols This scheme in principle requires a dynamic routing protocol that supports MD5 procedures. These currently include RIPv2, OSPF, ISIS, EIGRP and BGP4. Static routing is considered a special case and is serviced through the 'Trusted PE' model. The 'Trusted PE' model provides the ability for the Service Provider to authenticate routing information that is distributed between PE routers, without the requirement of MD5 authentication on the PE-CE links. This is useful when static routing is used between the PE and CE for example. Using this model, the PE router will export all routes from within a VRF that has been configured for authentication, using the previous draft-behringer-mpls-vpn-auth-01.txt page 5 Internet Draft MPLS VPN Authentication January 2003 described mechanisms. However, the check to see whether a route has been authenticated with the CE router is disabled. A receiving PE router is able to authenticate the routes using the local VRF key, which eliminates a misconfiguration of the route targets from causing any leak of routing information between VPNs. Since it is still possible for the Service Provider to a) misconfigure the export route targets AND b) misconfigure the key, the Trusted PE model is recommended only in exceptional cases. 6. The UPDATE-authenticator attribute The UPDATE-authenticator attribute is an optional, transitive BGP attribute, with an attribute type code value to be assigned. Its length is 20 octets, which is the length of the output of an MD5 function (16 octets), plus the 'generator' field, as shown in the following figure. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HMAC-MD5 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HMAC-MD5 (cont) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HMAC-MD5 (cont) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HMAC-MD5 (cont) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Generator | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 7. IANA Considerations The UPDATE-authenticator BGP attribute type will need to be registered with IANA, according to the procedures defined in [RFC2042]. 8. Security Considerations This modification to the behavior of the PE router aims at detecting inadvertent configuration mistakes of the Service Provider, and at isolating CE routers that appear not to belong to the VPN they were configured for. There is no protection against the Service Provider staff maliciously adding a CE router to a VPN. However, the malicious engineer must know draft-behringer-mpls-vpn-auth-01.txt page 6 Internet Draft MPLS VPN Authentication January 2003 the MD5 key of the VPN to be intruded. This threat can be avoided with CE-CE IPsec authentication, which is configured by the VPN customer, and to which the Service Provider does not have access. 9. Acknowledgements 10. References [RFC1771] "A Border Gateway Protocol 4 (BGP-4)". Y. Rekhter, T. Li. March 1995 [RFC2042] "Registering New BGP Attribute Types". B. Manning. January 1997. [RFC2082] "RIP-2 MD5 Authentication". F. Baker, R. Atkinson. January 1997. [RFC2104] "HMAC: Keyed-Hashing for Message Authentication". H. Krawczyk, M. Bellare, R. Canetti. February 1997. [RFC2154] "OSPF with Digital Signatures". S. Murphy, M. Badger, B. Wellington. June 1997. [RFC2385] "Protection of BGP Sessions via the TCP MD5 Signature Option". A. Heffernan. August 1998. [RFC2547] "BGP/MPLS VPNs". E. Rosen, Y. Rekhter. March 1999. 11. Author's Addresses Michael H. Behringer Cisco Systems, Inc. Avda de la Vega, 15; 28100 Alcobendas, Madrid; Spain Email: mbehring@cisco.com Jim Guichard Cisco Systems, Inc. 250 Apollo Drive Chelmsford, MA, 01824 Email: jguichar@cisco.com 12. Full Copyright Statement Copyright (C) The Internet Society (2000). All Rights Reserved. draft-behringer-mpls-vpn-auth-01.txt page 7 Internet Draft MPLS VPN Authentication January 2003 This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. 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